1. Field of the Invention
The present invention relates to a hydrodynamic bearing unit having superiority in high-precision rotation, high-speed stability, long durability, low noise and the like, and particularly, to a hydrodynamic bearing unit suitable for supporting a spindle of spindle motor used in information-processing equipment. The “spindle motor used in information-processing equipment” used herein includes a spindle motor for driving an optical disk such as CD-R/RWs and DVD-ROM/RAMs, a magneto-optical disk such as MOs, and a magnetic disk such as HDDs, a polygon scanner motor employed in a laser beam printer (LBP) or a copying machine, and the like.
2. Description of the Related Art
In recent years, a hydrodynamic bearing has been attracting much attention as a bearing for supporting a spindle of a spindle motor or a polygon scanner motor employed in the above-mentioned various information-processing equipment. The hydrodynamic bearing supports a spindle in a non-contact manner by utilizing hydrodynamic effect of lubricating oil generated in a bearing gap. Among those bearings utilizing such effect, as shown in FIG. 7, a hydrodynamic sintered lubricant-containing bearing as a bearing member 52 utilizing a sintered metal impregnated with lubricating oil or lubricating grease has advantageous features of low manufacturing cost and the like. Therefore, the bearing is expected to be used in further broader technological field as a bearing for the above-mentioned spindle motor or polygon scanner motor.
The hydrodynamic sintered lubricant-containing bearing is characterized in that lubricating oil circulates between a bearing gap C and the inner portion of the bearing member 52. Namely, oil oozes from the inner portion of the bearing member 52 due to the temperature increase of bearing unit or the pressure generation followed by the rotation of a spindle 51. Then, the oil is drawn to the bearing face via inclined hydrodynamic grooves 55 followed by the generation of a hydrodynamic effect. The oil is then returned to the inner portion of the bearing member 52 via micro pores of the bearing face by the positive pressure at the bearing face. Thereafter, again the oil oozes from the inner portion of the bearing member 52 to repeat this cycle.
In this case, the oil oozes not only from the inner circumferential surface of the bearing member 52 but also from the other surfaces, for example, an end surface 52a of the bearing member 52 on the opening side of a housing (in this case, the ooze is caused primarily by oil thermal expansion) Therefore, in some cases, the lubricating oil accumulates around the end surfaces 52a during operation. Particularly, when a ventilation path 59 is formed between the inner circumferential surface of a housing 53 and the outer circumferential surface of the bearing member 52 as an air exhaust path used in the assembling process, the oil oozing in a chamfer 52c of the bearing member 52 on the bottom side of the housing or on the surface of the ventilation path 59 also moves upwardly via the ventilation path 59. Accordingly, the amount of the accumulated oil around the end surface 52a on the opening side of the housing is apt to increase. In some cases, a gap 62 (an oil holding gap) between a sealing member 61 provided on the opening of the housing 53 and the end surface 52a of the bearing member 52 is filled with lubricating oil. When such condition occurs, the lubricating oil is in danger of leaking out of the bearing unit via the sealing gap between the inner circumferential surface of the sealing member 61 and the outer circumferential surface of the spindle 51.
To prevent such oil leakage, it is so important to prevent the oil accumulated in the oil holding gap 62 from touching the sealing member 61. Generally, in this kind of bearing, the bearing temperature during operation can be determined corresponding to operation conditions, and therefore, the touch between the oil surface and the sealing member 61 can be prevented by estimating the amount of volume increase due to thermal expansion in advance and subsequently, based on the estimation, designing the space volume of the oil holding gap 62 so as to be capable of accommodating the resultant total amount of oil including the estimated amount of oil volume increase.
However, even when the amount of the oil accumulated in the oil holding gap 62 is within the estimated volume, depending on various primary environmental conditions, the touch between the oil surface and the sealing member possibly occurs. For example, when a bubble remains in the ventilation path 59 after the insertion of spindle in the bearing unit, as shown in FIG. 8A, a bubble B is pushed away via the ventilation path 59 into the opening side of the housing during the operation, which is caused by the above-mentioned movement of the lubricating oil, and then expands wholly in the oil holding gap 62. The expansion of the bubble is performed in a popping manner and at the time of the popping, as shown in FIG. 8B, the bubble B touches the end surface of the sealing member 61, and as a result, the lubricating oil is possibly pushed away along the inner circumferential surface of the housing 53 to touch the end surface of the sealing member 61. Furthermore, depending on the physical orientation of motor during operation (in an inclined posture or horizontally placed posture), as shown in FIG. 9, the lubricating oil is possibly unevenly positioned and thus, the accumulated oil flows and touches the sealing member 61. In addition, such phenomenon as the amount variation of oil to be filled possibly becomes a primary cause of the above-mentioned touch.
When taking into account of those mechanisms described above, as a measure of the prevention of oil leakage, the measure of simply designing the width dimension of the oil holding gap 62 in a suitable value is not sufficient and therefore, as an additional measure to eliminate the above-mentioned complex and primary causes, an improved structure of bearing unit is expected to be developed to securely prevent the touch between oil surface and a sealing member.